The thesis deals with the question of how regulatory powers of the state are legally expressed, manifested and protected in international investment law (IIL). The saliency of this topic is evident in the continuing criticism of the international investment regime in academic, policy as well as public debate. The thesis first develops a conceptual framework for apprehending legal concepts in IIL which is based on (i) analysis of treaty interpretation with a particular focus on the object and purpose of international investment agreements (IIAs), (ii) function, nature and institutional characteristics of investment treaty arbitration and, finally, (iii) on general international law concepts expressing the regulatory aspect of state sovereignty, such as the notion of police powers. This conceptual framework is then applied to a case study of regulatory disputes under the fair and equitable treatment standard (FET), and analyses specifically the notion of legitimate expectations, procedural guarantees under FET, and the notion of reasonableness and proportionality. These legal concepts (police powers, legitimate expectations, proportionality, reasonableness) are analysed through their genealogy and historical evolution not only in IIL, but particularly in their original source environments, such as US constitutional law, comparative administrative law, or international human rights law. This thesis offers a more principled and reasoned set of guidelines for interpreting and applying investment treaty standards to regulatory disputes, although a specific application of the general framework was limited to regulatory disputes arising under the FET standard.